1. Field of the Invention
The present invention relates to radio frequency (RF) transponders, and more particularly, to a radio frequency identification (RFID) transponder that is powered by an internal battery and also has an ability to passively recover power from an interrogating RF field after the internal battery has become depleted.
2. Description of Related Art
In the automatic data identification industry, the use of RFID transponders (also known as RFID tags) has grown in prominence as a way to track data regarding an object to which the RFID transponder is affixed. An RFID tag generally includes a semiconductor memory in which digital information may be stored, such as an electrically erasable, programmable read-only memory (EEPROM) or similar electronic memory device. An RFID interrogator or reader may recover the digital information stored in the RFID tag using modulated radio frequency (RF) signals. One such communication technique is referred to as “backscatter modulation,” by which an RFID tag transmits stored data by reflecting varying amounts of an electromagnetic field provided by the RFID interrogator by modulating the antenna matching impedance of the tag. The RFID tag can therefore operate independently of the frequency of the energizing field, and as a result, the interrogator may operate at multiple frequencies so as to avoid RF interference, such as utilizing frequency hopping spread spectrum modulation techniques. Since RFID tags using backscatter modulation do not include a radio transceiver, they can be manufactured in very small, lightweight and hence inexpensive units.
RFID tags either extract their power from the electromagnetic field provided by the interrogator (also known as passive RFID tags), or include their own internal power source (e.g., battery). Passive RFID tags that extract their power from the interrogating field are particularly cost effective since they lack a power source, and can be constructed in smaller package sizes. A drawback of passive RFID tags is that they tend to require a shorter distance between the RFID tag and the RFID reader (known as the read range) in comparison to battery-powered RFID tags. Unlike passive RFID tags, battery-powered RFID tags have a limited life expectancy. Once the internal battery becomes depleted, the RFID tag becomes unusable. Since most batterypowered RFID tags are not designed to permit replacement of the battery, the RFID tag is usually discarded upon depletion of the battery and information stored in the RFID tag memory is irretrievably lost. In some cases, it may be critically important to recover the information stored in the RFID tag, thereby requiring the RFID tag to be disassembled to apply power to the electronic devices therein.
Accordingly, it would be desirable to provide an RFID tag having the attributes of both battery-powered and passively-powered tags. It would further be desirable to provide a battery-powered RFID tag that can be passively-powered after the battery becomes depleted so that stored information can be easily recovered.